Pregnancy Due Date Calculator
Calculate your estimated due date and track your pregnancy progress week by week with key milestones.
How Due Date is Calculated
The due date is calculated using Naegele’s Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14.
For irregular cycles, the calculator adjusts by the difference from 28 days. Conception date method adds 266 days, as pregnancy actually lasts about 38 weeks from fertilization.
Understanding Trimesters
- 1st Trimester (Weeks 1-12): Organ development begins. Morning sickness common. Highest miscarriage risk period.
- 2nd Trimester (Weeks 13-27): “Honeymoon period” with less nausea. Baby movements felt. Anatomy scan performed.
- 3rd Trimester (Weeks 28-40): Rapid baby growth. More frequent checkups. Prepare for delivery.
Important Prenatal Tests
- First Ultrasound (6-8 weeks): Confirms pregnancy, checks heartbeat, and dates the pregnancy.
- NT Scan (11-14 weeks): Screens for chromosomal abnormalities like Down syndrome.
- Anatomy Scan (18-22 weeks): Detailed check of baby’s organs, spine, and development.
- Glucose Test (24-28 weeks): Screens for gestational diabetes.
Important Considerations
- Due date accuracy: Only about 5% of babies arrive on their exact due date. Most are born within 2 weeks before or after.
- Ultrasound dating: First trimester ultrasound is most accurate for dating, within 5-7 days.
- Full term range: Babies born between 39-40 weeks are considered full term. 37-38 weeks is early term.
- Consult your provider: Your healthcare provider may adjust your due date based on ultrasound measurements.
Frequently Asked Questions
How is a pregnancy due date calculated from last period?
Naegele’s Rule adds 280 days to the first day of your last menstrual period and adjusts for cycle length: if your cycle is shorter or longer than 28 days, the calculator shifts the due date by the difference. A 30-day cycle shifts it 2 days later; a 26-day cycle shifts it 2 days earlier. This gives the estimated due date, which falls at 40 weeks of pregnancy.
How accurate is a pregnancy due date calculator?
Estimated due dates based on LMP are accurate to within about a week for women with regular cycles. Only approximately 5% of babies are born on their exact calculated due date. In clinical practice, an early ultrasound (8–12 weeks) provides a more accurate due date by measuring the embryo’s actual size. The calculator’s date is a useful planning estimate, not a guarantee.
How is due date calculated after IVF embryo transfer?
For IVF pregnancies, the calculator uses the embryo transfer date and the embryo’s developmental stage. A day-5 blastocyst transfer adds 261 days to get the due date, because the embryo is already 5 days old. A day-3 embryo transfer adds 263 days. This is more precise than LMP-based calculation because the fertilization date is known exactly.
What are the pregnancy trimester date ranges?
The first trimester spans weeks 1–12, the second trimester covers weeks 13–26, and the third trimester runs from week 27 to birth around week 40. The calculator identifies which trimester you are currently in and shows key milestones: first ultrasound around week 8, NT scan at week 12, anatomy scan at week 20, glucose test at week 26, and full-term status starting at week 37.
What if my last period date is unknown — can I still calculate a due date?
Yes. The calculator offers two alternative methods if you do not know your LMP: the conception date method (adds 266 days from the date of conception) or the IVF transfer date method. If you are unsure of all three, the most reliable option is a first-trimester ultrasound, which your healthcare provider can order to establish gestational age and due date.